1. Field of the Invention
The present invention relates to a fuel cell system capable of sealing a cathode by closing a first shut-off valve and a second shut-off valve after stopping electro-chemical reaction progressing in a fuel cell; and capable of releasing the cathode by opening the first shut-off valve and the second shut-off valve during the electro-chemical reaction progressing in the fuel cell.
2. Description of the Related Art
Japanese Patent Laid-open Publication No. 2008-218072 (hereinafter called Patent Document 1) discloses an example of fuel cell system including a fuel cell having an air-inlet shut-off valve provided to an inlet of a cathode and an air-outlet shut-off valve provided to an outlet of the cathode and preventing gas crossover, i.e. a short circuit caused by either gas passing from one side to the other side of the fuel cell through a membrane while no electro-chemical reaction is under way in the fuel cell, by sealing the cathode by closing the air-inlet shut-off valve and the air-outlet shut-off valve after stopping the electro-chemical reaction progressing in the fuel cell.
The fuel cell system disclosed in Patent Document 1 has a buffer tank for storing air compressed by an air compressor and having a higher pressure than that of air supplied to the fuel cell; and switches the air-inlet shut-off valve and the air-outlet shut-off valve between open state and closed state by using pressure difference between an atmospheric pressure chamber communicating with ambient air and a chamber storing pressurized air supplied from the buffer tank.
Japanese Patent Laid-open Publication No. 2006-221836 (hereinafter called Patent Document 2) discloses a fuel cell system including a fuel cell having two normally-closed electromagnetic valves each provided to an outlet and an inlet of a cathode; and closing the electromagnetic valves while electro-chemical reaction is kept from progressing in the fuel cell.
However, the prior art fuel cell system disclosed in Patent Document 2 has a disadvantage that power output efficiency may be reduced because the fuel cell system uses solenoid valves to which electricity must be supplied continuously while they are in open state. In particular, the fuel cell has to bear more reduced power output efficiency if greater electricity is necessary for opening the solenoid valves.
The prior art of Patent Document 1, disclosing that each shut-off valve is switched by using air pressure (i.e. pneumatically) supplied from the buffer tank, may have another disadvantage that the shut-off valves, which must be opened during electro-chemical reaction in the fuel cell, may be closed if air pressure for maintaining the shut-off valves in open state fluctuates or is reduced for some reasons. Such abnormal state of air pressure in the fuel cell system may result in damage to the fuel cell system or system down of the fuel cell system.
The prior art fuel cell system disclosed in Patent Document 1 is also disadvantageous due to its complexity and large size since a control mechanism for opening and closing the shut-off valves uses many parts including a buffer tank; valves for switching air supplied from the buffer tank to each shut-off valve; a shut-off valve for switching air supplied to the buffer tank; and pressure sensors for detecting the pressure in the buffer tank and the likes.
In consideration of a pressure drop which may occur when air pressure for maintaining the open state of shut-off valves fluctuates due to air leakage from air flowpaths or fluctuation of ambient temperature, the prior art fuel cell system of Patent Document 1 needs another control mechanism, e.g. an air-supply mechanism for maintaining air pressure supplied to the shut-off valves in open state.